A study published in Science Advances finds that smoke from the Eaton Fire, which tore through Altadena on 7 January 2025, carried airborne lead roughly 80 times above normal atmospheric levels, exposing about 7 million Los Angeles residents to concentrations above 50 nanograms per cubic meter on 9 January. Led by atmospheric chemists Haroula Baliaka and Nga Ng at the California Institute of Technology, the research attributes the toxic load to the urban fuel burned: homes, vehicles and infrastructure rather than vegetation.
When Santa Ana winds pushed flames from Eaton Canyon into Altadena on 7 January 2025, the Eaton Fire became one of the most destructive blazes in California's history. New research shows its smoke was also extraordinarily toxic. By 9 January, plumes blanketed about 7 million Los Angeles residents in air laden with lead roughly 80 times above normal atmospheric levels for a 24-hour period.
The study, published in Science Advances and led by atmospheric chemists Haroula Baliaka and Nga Ng at the California Institute of Technology (Caltech), found the smoke also carried high concentrations of chlorine, bromine, zinc, copper and arsenic. More than 9,000 structures burned, along with electronics and vehicles, and those fuels gave the plume a chemistry unlike that of a typical wildland fire.
"Unlike the wildfire smoke that has been the focus of decades of air quality research, this plume originated primarily from burning homes, vehicles, and infrastructure, a fundamentally different fuel with a distinct chemical signature," the team writes.
Real-time readings came from a Caltech air composition monitoring facility in Pico Rivera, directly south of the fire. Air quality was at its worst in the first two days, with eastern Los Angeles and the San Gabriel Valley recording the highest 24-hour average lead concentrations.
"Under typical conditions, daily lead at Pico Rivera averages just 0.6 nanograms per cubic meter. Overlaying daily lead maps with census data reveals the scale of exposure: On 9 January, approximately 7 million residents experienced concentrations exceeding 50 nanograms per cubic meter," Baliaka and the team report. They add that as peak concentrations declined with the transition from structural to wildland burning, the total population exposed continued to rise as the plume dispersed across the basin.
Similar results have been reported separately by a University of California, Los Angeles team using monitoring stations on its Westwood campus and in Huntington Park. Compared with a wildfire that burned almost exclusively in vegetated wildlands in Riverside in 2024, the Eaton plume contained 50 times the concentration of lead, 7 times the halogens chlorine and bromine, 4 times the zinc, and 3 times the copper and arsenic. Many Altadena houses are early to mid 20th century builds, dating from an era when lead paints and brominated flame retardants were commonly used.
The health consequences for people exposed to the smoke remain unknown. The researchers are concerned the contaminants may persist long after the fires, having settled across playgrounds, watersheds and farmlands, and make an urgent call for epidemiological studies, particularly in children and other vulnerable populations. They note that while the toxicity of chronic exposure to individual elements is well established, far less is known about short-duration, high-intensity exposures and the cumulative effects of simultaneous exposure to complex mixtures.
"As climate change and urban expansion drive more fires into cities, the question is not whether another urban firestorm will occur," they warn. "What remains to be seen is what we will have learned from the previous ones and whether monitoring and emergency response systems can adapt fast enough to characterize these emissions and their exposure risk."
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Concentration of pollutants in Eaton fire smoke compared to a typical wildland fire
times the concentration in wildland fire
Eaton plume concentrations relative to a 2024 Riverside vegetated wildland fire. Halogens are chlorine and bromine; copper and arsenic are reported together. Source: Baliaka, Ng et al. (Caltech) / Science Advances
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Analysis
Why This Matters
The exposure did not end when the flames were out: the researchers warn contaminants have settled across playgrounds, watersheds and farmlands and may persist in soil and waterways.
The health effects of short, intense exposure to this mixture of metals and halogens are largely unknown, and the team says epidemiological studies are urgently needed, particularly for children and other vulnerable populations.
Climate change and urban expansion are expected to drive more fires into cities, making the findings relevant well beyond Los Angeles.
Background
Wildfire smoke has been the focus of air quality research for decades, but the Eaton plume came from a different fuel: burning homes, vehicles and infrastructure. Many Altadena houses date from the early to mid 20th century, when lead paints and brominated flame retardants were common, which may explain the plume's toxicity. A separate UCLA team, monitoring in Westwood and Huntington Park, reported similar results, and comparisons with a 2024 Riverside wildland fire showed the Eaton plume carried far higher concentrations of lead, chlorine, bromine, zinc, copper and arsenic.
What to Watch
Whether the call for epidemiological studies is taken up, with health outcomes tracked in children and other vulnerable populations.
How quickly monitoring and emergency response systems can characterize emissions when the next urban fire occurs.
Long-term testing of soil and waterways, as contaminants that settled across the Los Angeles basin could persist for years.
11 claims in this story checked against 1 source: 11 supported.
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SupportedThe Eaton Fire smoke exposed about 7 million Los Angeles residents to lead roughly 80 times above normal atmospheric levels.ScienceAlert: “By January 9, plumes of smoke blanketed around 7 million Los Angeles residents in air laden with lead roughly 80 times above normal atmospheric levels for 24 hours.”
SupportedOn 9 January, about 7 million residents experienced lead concentrations exceeding 50 nanograms per cubic meter.ScienceAlert: “On 9 January, approximately 7 million residents experienced concentrations exceeding 50 nanograms per cubic meter,”
SupportedThe study was published in Science Advances and led by atmospheric chemists Haroula Baliaka and Nga Ng at Caltech.ScienceAlert: “That’s one of the major findings from a new study published in Science Advances, led by atmospheric chemists Haroula Baliaka and Nga Ng at the California Institute of Technology (Caltech).”
SupportedSanta Ana winds pushed flames from Eaton Canyon into Altadena on 7 January 2025.ScienceAlert: “When the Santa Ana wind pushed flames from Eaton Canyon into Altadena on 7 January 2025, it quickly escalated into one of the most destructive fires in California's history.”
SupportedThe smoke contained high concentrations of chlorine, bromine, zinc, copper and arsenic.ScienceAlert: “As well as lead, the smoke contained high concentrations of chlorine, bromine, zinc, copper, and arsenic.”
SupportedThe researchers wrote that the plume came primarily from burning homes, vehicles and infrastructure, giving it a distinct chemical signature.ScienceAlert: ““Unlike the wildfire smoke that has been the focus of decades of air quality research, this plume originated primarily from burning homes, vehicles, and infrastructure, a fundamentally different fuel with a distinct chemical signature,” writes the team.”
SupportedMore than 9,000 structures burned, along with electronics and vehicles.ScienceAlert: “More than 9,000 structures in the area burned, along with electronics and vehicles.”
SupportedUnder typical conditions, daily lead at Pico Rivera averages 0.6 nanograms per cubic meter.ScienceAlert: “Under typical conditions, daily lead at Pico Rivera averages just 0.6 nanograms per cubic meter.”
SupportedCompared with the Riverside wildfire, the Eaton plume contained 50 times the lead, 7 times the chlorine and bromine, 4 times the zinc, and 3 times the copper and arsenic.ScienceAlert: “Compared to the Riverside fire, the Los Angeles Eaton plume contained 50 times the concentration of lead, along with 7 times the concentration of the halogens chlorine and bromine, 4 times the levels of zinc, and 3 times the levels of copper and arsenic.”
SupportedThe health consequences for people exposed to the smoke remain unknown.ScienceAlert: “The health consequences for people who were exposed to this smoke remain unknown.”
SupportedThe researchers warned that it is unknown whether monitoring and emergency response systems can adapt fast enough to characterize emissions and exposure risk from future urban firestorms.ScienceAlert: ““What remains to be seen is what we will have learned from the previous ones and whether monitoring and emergency response systems can adapt fast enough to characterize these emissions and their exposure risk.””